Conventional stands in medical use offer the possibility of manually positioning various diagnostic or therapeutic instruments in up to six degrees of freedom. A manual stand of this kind is described, e.g., in European Patent 0,023,003. However, a series of disadvantages for the surgeon results from manual operation. Thus, inadvertent erroneous movements require a subsequent time-consuming and complicated correction. It is furthermore found to be disadvantageous that in each case mechanically balanced systems are exactly balanced only for a defined instrument weight. This means that a change of the instrument weight requires a rebalancing of the stand system with an accuracy of about 100 g. Such a rebalancing during an operation, after a possible instrument change, is extremely troublesome and complicated. Furthermore, even with well balanced mechanical stand systems, the acceleration or braking of the instrument motion requires a certain operating force proportional to the acceleration or braking, which likewise has a troublesome effect during microsurgical work. Finally, no stereotactic work is in general possible with stands which are moved manually, since the momentary working field is not obtained with usual stands.
An operation microscope is known from German Offenlegungsschrift 4,032,207, which is arranged on a motor-driven, multi-jointed mechanism. Thus, a limited stereotactic use of the operation microscope is possible by means of suitable path and angle detectors. It is considered disadvantageous in this system, however, that no safety measures are provided against a possible failure of the operating electric supply or of the path and angle detectors coupled to it. Thus the multi-jointed mechanism shown has no mechanical balancing that could stabilize the system on failure of the drive units so that at least a limited further procedure could be possible. In addition, with a possible breakdown of the drives, a failure of the path or angle detectors coupled to the drives also results, i.e., a complete system failure potentially results during an operation.